The invention relates to an apparatus for detecting and discriminating phase jumps of a predetermined value .DELTA..phi..sub.e in a periodic electric signal of frequency f.sub.e contained in an electric input signal e(t).
The general problem is that in a periodic electric signal phase jumps of predetermined values must be detected, these values frequently being sub-multiples of .pi.. In the field of communications, phase modulation is one of the means for conveying information. In that case, the phase jumps are produced at a high rate and equipment such as modems are specially designed for transmitting, receiving or generating this type of signal.
The invention relates more specifically to recognizing periodic signals containing phase jumps, preferably a unique square-wave or sinusoidal signal, which is located in the voice frequency band and whose phase jumps, of a predetermined value comprised between 0.degree. and 360.degree., are produced at regular intervals. Detecting these phase jumps enables the remote control of apparatus in, for example, electric or telephone networks.
In the field of telephone communication, the specific problem is the echo of electrical origin. The echo is generally the result of an impedance mismatch at the various 2-wire or 4-wire junctions of tandem links forming a telephone connection, or an imperfect decoupling between the 4-wire receive and send ports of 2-to-4-wire junctions, defining thereby the balancing attenuation, or these two phenomena combined. The trouble caused by an echo becomes more important as its level is higher and its delay is longer.
To obviate the electrical echo in circuits having a long propagation time, two types of equipment are the subject of CCITT Recommendations: the echo suppressors dealt with in Recommensation G.164, CCITT Yellow Book edition 1981, Volume III.1, page 154 ff. and the echo cancellers or compensators described in Recommensation G.165, CCITT Yellow Book edition 1981, Volume III.1, page 182 ff. The echo cancellers constitute an improvement with respect to echo suppressors because of the fact that they act less abruptly and that, for example, in case of a speech signal, they render it possible to reduce at the receiving end, the level of the echo signal whilst retaining the far-end speech information signal at its normal level.
Besides its regular use for speech signals whose terminals are subscriber sets, the telephone network is increasingly used for transmitting data signals, either in half-duplex or in full-duplex, and the terminals are then data modems. The specific character of the data signals, notably the existence of a level which is the same on transmission and on reception, renders the transmission of these signals incompatible with the presence of echo suppressors in the 4-wire telephone lines. For that reason, the Recommendation G.164 of the above-mentioned CCITT Yellow Book provides for a disabler in each echo suppressor. This disabler should be responsive to a disabling tone transmitted by the data transmission modem at the pure frequency, i.e. the frequency without phase jumps, of 2100 Hz.+-.21 Hz.
When echo cancellers, alternatively designated echo compensators, appeared on the market, the CCITT reacted by recommending, in Recommendation G.165 of the above-mentioned Yellow Book, the same standard for disabling echo cancellers as the previously defined standard for echo suppressors, namely transmission of and making responsive to a disabling tone at the pure frequency of 2100 Hz.+-.21 Hz.
It has been found that the presence of the echo canceller in the line significantly improved the data transmission in the half-duplex mode, but then the problem occurs that the echo suppressors must be disabled without disabling the echo cancellers, taking account of the fact that for full-duplex data transmission the same disabling signal must disable both the echo suppressors and the echo cancellers which may be present in the 4-wire part of the line. The solution to this problem, as recommended by Recommendation G.165 of the CCITT Red Book edition of 1984 which is actually still valid, is to have the echo canceller selectively recognize a disabling tone of a frequency of 2100 Hz.+-.21 Hz containing periodic phase inversions produced every 450.+-.25 ms, which cause the echo canceller to be disabled. In contrast the echo canceller must not respond to the presence on the line of a disabling tone of the pure frequency of 2100 Hz.
Thus, for speech signal transmission, in the absence of a 2100 Hz disabling tone, the echo suppressors and the echo cancellers are operative. For the data transmission in the half-duplex mode, the sending half-duplex data modem transmits a disablinag tone at the frequency of 2100 Hz, which disables the echo suppressors but in no way affects the operation of the echo canceller.
For the full-duplex transmission of data, the sending full-duplex data modem transmits a 2100 Hz disabling signal, with phase inversions, which disables both any echo suppressors and any echo cancellers present. In the latter case, each modem has its own echo attenuation equipment.